1. Technical Field of the Invention
This invention relates generally to electromagnetic transducers such as audio speakers, and more specifically to a geometry having a low reluctance return path for magnetic flux from a secondary drive magnet.
2. Background Art
Speakers are shown in cross-section in this document. Because speakers are generally cylindrically or rotationally symmetrical about an axis line or center line, only one side of any given speaker is shown, but the skilled reader will readily appreciate the three-dimensional structure which is thus represented. The reader will appreciate, however, that the invention is not limited to such axially symmetric implementations.
FIG. 1A illustrates a conventional audio speaker 10 such as is known in the prior art, shown as symmetrical about a center line CL. The speaker includes a magnetically conductive pole plate 12 which includes a pole 14 which may be either coupled to or integral with the base 16 of the pole plate, as shown. The pole may include an axial hole 18 for permitting airflow to cool the motor structure and depressurize the diaphragm assembly. A ring-shaped permanent magnet 20 surrounds the pole, with a cavity 22 between them. A magnetically conductive top plate 24 surrounds the pole, with a magnetic air gap 26 between them. Typically, the magnetic air gap will be smaller than the cavity. The pole plate, magnet, and top plate may collectively be termed a magnet assembly or a motor structure. The heavy black arrows denote exemplary directions of flux flow, throughout this document; the skilled reader will readily appreciate that the magnets may be reversed, and the flux will flow the opposite direction, and the transducer will operate correctly, especially when provided with an inverse phase electrical input signal.
An electrically conductive voice coil 28 is rigidly attached to a cylindrical bobbin or voice coil former 30. The voice coil is suspended within the magnetic air gap to provide mechanical force to a diaphragm 32 which is coupled to the bobbin. When an alternating current is passed through the voice coil, the voice coil moves up and down in the air gap along the axis of the speaker, causing the diaphragm to generate sound waves.
A frame 34 is coupled to the magnet assembly. There are two suspension components. A damper or spider 36 is coupled to the bobbin and the frame, and a surround 38 is coupled to the diaphragm and the frame. These two suspension components serve to keep the bobbin and diaphragm centered and aligned with respect to the pole, while allowing axial movement. A dust cap 40 seals the assembly and protects against infiltration of dust particles and other stray materials which might contaminate the magnetic air gap and thereby interfere with the operation or quality of the speaker.
When, as shown, the voice coil is taller (along the axis) than the magnetic air gap, the speaker is said to have an “overhung” geometry. If, on the other hand, the voice coil were shorter than the magnetic air gap, the speaker would be “underhung”.
If the voice coil moves so far that there exists a different number of voice coil turns within the air gap (i.e. an overhung voice coil has moved so far that one end of it has entered the air gap, or an underhung voice coil has moved so far that one end of it has left the air gap), the speaker begins to exhibit nonlinear characteristics, and the sound quality is distorted or changed. This is especially problematic when playing low frequency sounds at high volume, which require maximum voice coil travel.
The common approach to solving this problem has been to use highly overhung or highly underhung geometries to achieve a high degree of linear voice coil travel. These approaches have inherent limitations, however. The highly overhung motor requires increasingly longer coils, which in turn increases the total moving mass of the diaphragm assembly. At some point, this ever-increasing mass becomes so great that the inherent mechanical design limits are reached, which prevents any further controllable increase in excursion. At the same time, increasing the voice coil mass with no resultant increase in utilized magnetic flux will reduce the overall efficiency of the transducer. Efficiency is proportional to BL squared, and inversely proportional to mass squared. In the highly underhung geometry, other practical limits are reached because of the relative increase in magnet area required to maintain a constant B across the magnetic gap height in order to achieve higher linear excursions without sacrificing efficiency. Unfortunately, this increase in available magnetic flux, B, does not result in an increase in BL, and therefore the transducer's efficiency also does not increase.
One hybrid approach has been to provide the bobbin with two tandem voice coils which travel in two respective magnetic air gaps, such as is taught in U.S. Pat. Nos. 4,783,824 to Kobayashi and 5,740,265 to Shirakawa. These are both “push-pull” geometries, in which the magnetic flux over the top magnetic air gap travels in the opposite direction as the flux over the bottom magnetic air gap; this requires that the two voice coils be wound in opposite directions, and it requires twice the total voice coil length and a longer bobbin without increasing the total linear excursion, all of which add manufacturing cost with minimal benefit. Kobayashi further teaches that the voice coils may be wound in the same direction if the currents through them are of opposite phases. Unfortunately, this requires each voice coil to have its own, dedicated pair of electrical inputs, which further increase the complexity and cost of the transducer.
In the prior art overhung speakers, 100% of the magnetic air gap is always active during linear operation. In the prior art underhung speakers, 100% of the voice coil windings are always active during linear operation.
Speakers may generally be classified as having an external magnet geometry (in which ring magnets surround a pole plate) or an internal magnet geometry (in which a cup contains magnets). Pole plates and cups may collectively be termed magnetic return path members or yokes, as they serve as the return path for magnetic flux which has crossed over the magnetic air gap.
Materials may be classified as either magnetic materials or non-magnetic materials. Non-magnetic materials may also be termed non magnetically conductive materials; aluminum and chalk are examples of non-magnetic materials. Magnetic materials are classified as hard magnetic materials and soft magnetic materials. Hard magnetic materials are also called permanent magnets, and generate magnetic flux fields without outside causation. Soft magnetic materials are those which, although not permanent magnets, will themselves become magnetized and generate flux in response to their being placed in a magnetic field. Soft magnetic materials include the ferrous metals such as steel and iron.
One problem with the prior art geometries is leaking magnetic flux (denoted FL).
FIG. 1B illustrates a shielded speaker 11 which includes a pole plate 12, a primary magnet 20, a primary plate 24, and other components as shown in FIG. 1A, with an additional shielding or bucking magnet 13. The bucking magnet is located on the opposite side of the pole plate from the magnet assembly, and serves to buck or cancel out the leaking flux. A shield 15 encloses the magnet assembly and the bucking magnet, and further reduces flux leakage.
FIG. 1C illustrates the speaker 17 taught in U.S. Pat. No. 5,550,332 to Sakamoto. The speaker includes a primary drive magnet 19, a drive plate 21, a bucking magnet 23, and a magnetically conductive outer ring 25. The drive plate and outer ring define a magnetic air gap 27. The bucking magnet is positioned on the opposite side of the drive plate from the primary magnet, and is oriented with its polarity opposite that of the primary magnet. In this geometry, the bucking magnet is not used to reduce flux leakage (and, in fact, it increases flux leakage)—it is used to increase flux density over the magnetic air gap. The magnet assembly components are held in place by a non-magnetically-conductive holder 29. The magnetic flux return paths from the outer ring to both magnets are solely via leakage flux FL.
FIG. 1D illustrates the speaker 31 taught in U.S. Pat. No. 4,783,824 to Kobayashi. The speaker includes a pole piece 12, a primary drive magnet 20, a first drive plate 24, and a diaphragm assembly substantially as in FIG. 1A. A bucking magnet 33 is positioned on the opposite side of the primary drive plate, with its poles oriented opposite those of the primary drive magnet, as in Sakamoto. Rather than relying on high reluctance air return paths for the magnetic flux from the primary drive plate to the respective magnets, Kobayashi adds a second drive plate 35 which defines a second drive magnetic air gap 37, in which Kobayashi places a second voice coil 39. The Kobayashi speaker is thus a “push-pull” geometry, with the respective voice coils either wound in opposite directions or driven with opposite-phase alternating current electrical signals.
What is needed is a speaker geometry which provides a low reluctance return path for flux to the bucking magnet, without requiring a push-pull voice coil arrangement.